UML Design Tools
Several tools exist for creating UML, or class diagrams, preparatory to coding a project. Online tools such as Drawio are great! However, for those with a preference to use in-editor tools I have found the extensions below to be useful.
Umlet by The Umlet Team
Umlet allows users to create a wide array of useful diagrams. So far, my primary application of this tool has been in designing class diagrams for relatively basic C# programs while at University. For example, the below class diagram is for a mindfullness activities app: